1. Field of the Invention
This invention relates to an induction heating apparatus, and more particularly to an induction heating apparatus which detects abnormality in a case where, although an iron cooking-pot essentially is to be placed as a load on a heating unit, an improper load, such as an aluminum or copper pot or a utensil, such as a spoon, other than the pot, is placed on the heating unit, the iron pot, even when used, is placed far apart from the center of the same, or an abnormal load, such as no load, occurs, thereby inhibiting oscillation of an inverter circuit for induction heating apparatus.
2. Description of the Prior Art
For the purpose of discriminating the proper load condition from the abnormal load condition of the aforesaid improper load, smaller load or no load, an apparatus has hitherto been known which detects an input current and compares it with the predetermined reference level to thereby carry out the above discrimination. Such apparatus, in respect of the input setting for heating possible in a range of 200 to 1200 W, need to be set to the reference level less than 200 W in conformation to the lowest set-input. Hence a problem occurred in that in a case of a larger input set value, the discrimination sensitivity is remarkably low. For example, there has been the problem in that even when the iron pot is shifted from the center of cooking surface during the heating, the increment of input is small, whereby the oscillation of an inverter circuit does not stop. Also, the apparatus, which detects an input to the load, takes considerably much time until the detection and cannot quickly protect a switching element of the inverter circuit or the like, whereby the switching element may have been damaged due to an overload.
The induction heating apparatus, which has solved the above problems, is disclosed in the Japanese Patent Publication No. 44061-1978, which comprises an input adjusting coil of variable inductance connecting in series with an induction heating coil, the input adjusting coil being adapted to be adjusted in association with a circuit element (variable resistor) for setting the reference level. Such construction, however, often suffers the energy loss due to the input adjusting coil, thereby having not at all the practicability.